This invention relates to a hydraulic brake system for automotive vehicles, with at least one load sensor sensing the load condition of the vehicle and with at least one sensor associated with each vehicle axle for the detection of a braking parameter decisive for the braking operation, with the measured values sensed being supplied to an electric evaluation circuit which during the braking operation permanently computes suitable braking pressure values for the wheel brakes of the rear axle and delivers corresponding control signals to a braking pressure modulator.
European Patent Publication (EP) No. 0 062 246 A1 discloses a brake system which features the above characteristics. In this publication a hydraulic brake system is described where at least at the rear axle of the automotive vehicle a sensor is arranged which registers the respective load conditions of the vehicle and delivers a corresponding electric signal to an electric evaluation circuit. A tandem master cylinder is used in this known brake system. A first chamber of the tandem master cylinder pressurizes the wheel brakes of the front axle of the automotive vehicle while, by way of a braking pressure modulator, and a second working chamber hydraulically communicates with the wheel brakes of the rear axle. Further, the brake system has measuring devices which register the respective pressure prevailing in the wheel brakes and transform it into electric signals which likewise are supplied to the electric evaluation circuit.
A pressure control valve serves for modulation of the hydraulic braking pressure in the wheel brakes of the rear axle of the automotive vehicle. Said pressure control valve reduces the pressure supplied to the rear wheel brakes relative to the pressure in the front wheel brakes in accordance with a predetermined characteristic line. In addition, the output signal of the evaluation circuit is supplied to the pressure control valve. Thus one may vary the control behavior of the braking pressure modulator largely at discretion in dependence on the respective marginal conditions present.
As becomes evident from the above, the output signal of the evaluation circuit is decisively dependent on the hydraulic pressures generated in the wheel brakes. This, however, is disadvantageous in that it is impossible to absolutely avoid overbraking of the rear axle of the automotive vehicle; it being assumed, that the front wheel brakes are pressurized by a hydraulic pressure which leads to a lock-up of the vehicle wheels associated with the front wheel brakes. In accordance therewith, the evaluation circuit computes a hydraulic pressure for the rear wheel brakes which admittedly is reduced as compared with the pressure in the front wheel brakes, which, however, may be above the lock-up pressure.
It is thus an object of the present invention to further develop a hydraulic brake system of the type referred to so as to positively avoid a lock-up of the rear wheels of the automotive vehicle.